This program is a study of those responses of the vascular system that are directly or indirectly related to the level of heat energy input to an organism. The responses include changes in morphology and variations in local and regional blood flow. Possibly related to both of these categories of response, and certainly to the latter are those components of the peripheral and central nervous system that respond to thermal stimuli, detect the thermal status throughout the body, and are responsible for appropriate effector responses. Also it is proposed to continue a study of the effects of heat energy on vascular growth, and to investigate possible applications of these effects to clinical problems relating to cardiovascular pathology. The instrumentation and procedures employed at the same time lend themselves to a qualitative and quantitative study of the changes in local and regional blood flow occurring in tissues during thermoregulatory adjustments to changes in environmental and metabolic heat input. Efforts during the current year will include an investigation of possible intra-abdominal thermoreceptors in the cat with respect to their influence on the thermoregulatory responses of panting, shivering and possibly regional blood flow changes.